Thread inserts having a generally helical shape are commonly used to create a receiving thread for subsequent insertion of a screw or bolt into a relatively soft receiving material such as polymeric material or soft metals such as aluminum or magnesium, etc. An aperture is pre-tapped in the receiving material to receive the helical insert. The helical insert or “coil” includes a plurality of both outside and inside surface areas acting as screw threads. A tang which is an extension of the coil extends radially inwardly from a coil section at a leading end of the coil and is provided for grasping by an insertion tool. Using the insertion tool and tang, the coil is threaded into the aperture, the coil outer surfaces engaging the tapped threads of the aperture. The tang can then be broken off by material failure at a notch pre-formed at a junction of the tang and the helical coil. The coil inner surfaces then provide engagement surfaces for receiving the threads of a screw, bolt or other fastener.
Known helical coil designs provide retention strength or resistance to removal by sheared ends provided at both ends of the coil which bite into the softer receiving material when removal of the coil is attempted or a fastener is rotated into the coil. To further improve coil retention, “insert ends” having no chamfers together with the sheared ends are used. Removal of coils is often required, for example to remove and/or replace the fastener during maintenance, or if the coil is damaged or is installed improperly. During coil removal, the softer receiving material is susceptible to damage. When the receiving material is a casting or complex part, the cost to replace the damaged part can be excessive.
Coil designs are known which assist in removal of the coil, for example U.S. Pat. No. 6,171,040 to Sato. Sato discloses a pair of notches 10 which are formed at a back end portion of the coil and include contact surfaces 10b for engagement by a tool such as a bladed screw driver. At the back end of the coil, Sato also discloses a center flat surface 20 having chamfered surfaces 15, 16, 17 and 18. The disadvantage of the Sato and other known designs is that damage to the internal threads of the receiving material aperture can still occur when the coil is backed out, due to the corners of the chamfer geometry at the back end of the coil.